JP7246477B2 - Polarizing film, polarizing plate, and method for producing the polarizing film - Google Patents

Description

The present invention relates to a polarizing film, a polarizing plate, and a method for producing the polarizing film.

A liquid crystal display device, which is a typical image display device, has polarizing films arranged on both sides of a liquid crystal cell due to its image forming method. As a method for producing a polarizing film, for example, there is a method in which a laminate having a resin substrate and a polyvinyl alcohol (PVA) resin layer is stretched and then dyed to obtain a polarizing film on the resin substrate. It has been proposed (for example, Patent Document 1). According to such a method, since a thin polarizing film can be obtained, it has attracted attention as a potential contribution to the thinning of image display devices in recent years.

Moreover, in recent years, the market for wearable devices such as smart glasses and smart watches is expanding. If the wearable device is of a type that is attached to the body (e.g., skin) or clothing, the polarizing film mounted on the device can also follow the stretch of the body or clothing. is required. However, conventional polarizing films are generally easy to tear (break) along the absorption axis direction and have substantially no stretchability.

JP-A-2001-343521

The present invention has been made to solve the conventional problems described above, and its main purpose is to provide a polarizing film that can follow the elongation of the body (e.g., skin) and clothing.

According to one aspect of the present invention, the film is composed of a polyvinyl alcohol resin film containing a dichroic substance, and has a strain amount of 10% or more when pulled in the absorption axis direction at a load change rate of 98.0 mN/min. and a shrinkage rate of 5% or less in the absorption axis direction when heated at 85° C. for 120 minutes.
In one embodiment, the polarizing film has a thickness of 8 μm or less.
In one embodiment, the polarizing film has a single transmittance of 40.0% or more and a degree of polarization of 99.0% or more.
According to another aspect of the present invention, there is provided a polarizing plate having the polarizing film and a protective layer disposed on at least one side of the polarizing film.
According to another aspect of the present invention, in the method for producing a polarizing film, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of a long thermoplastic resin substrate. Drying to shrink 2% or more in the width direction by heating the laminate while conveying it in the longitudinal direction by performing an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a laminate. and shrinkage treatment in this order, wherein the total stretching ratio of the air auxiliary stretching treatment and the underwater stretching treatment is 2.5 to 4.5 times the original length of the laminate. , a manufacturing method is provided.

According to the present invention, there is provided a polarizing film in which breakage along the absorption axis direction is suppressed and the amount of strain in the absorption axis direction is equal to or greater than a predetermined value. Conventionally, it has been difficult to obtain practically acceptable optical properties (typically, single transmittance and degree of polarization) with such stretchable polarizing films. and practically acceptable optical properties.

1 is a schematic cross-sectional view of a polarizer according to one embodiment of the invention; FIG. FIG. 4 is a schematic diagram showing an example of drying shrinkage treatment using a heating roll. 4 is a graph showing strain amounts and heat shrinkage rates of polarizing films produced in Examples and Comparative Examples.

Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

A. Polarizing Film The polarizing film according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing a dichroic substance, and has a strain amount of It is 10% or more, and the shrinkage rate in the absorption axis direction when heated at 85° C. for 120 minutes is 5% or less. With such a configuration, it is possible to remarkably suppress the polarizing film from tearing (breaking) along the absorption axis direction, and to impart stretchability in the absorption axis direction. As a result, a stretchable polarizing film (resulting in a polarizing plate) can be obtained. Such a polarizing film (and as a result, a polarizing plate) can be preferably applied to devices that require stretchability, such as wearable devices that are worn by being attached to the body, clothing, or the like.

The strain amount (elongation rate) of the polarizing film when pulled in the absorption axis direction at a load change rate of 98.0 mN/min is 10% or more, preferably 15% or more, and more preferably 20% or more. If the strain amount is within such a range, a polarizing film having stretchability in the direction of the absorption axis can be obtained. The upper limit of the strain amount can be, for example, 100%.

The shrinkage ratio of the polarizing film in the absorption axis direction when heated at 85° C. for 120 minutes is 5% or less, preferably 4.5% or less, and more preferably 4.0% or less. If the shrinkage ratio is within such a range, a polarizing film in which breakage along the absorption axis direction is suppressed can be obtained.

The strain amount (elongation rate) of the polarizing film when pulled in a direction orthogonal to the absorption axis direction (transmission axis direction) at a load change rate of 98.0 mN/min is, for example, 10% or more, preferably 15% or more, More preferably, it is 20% or more. If the amount of strain is within this range, it is possible to obtain a polarizing film that is prevented from breaking along the direction of the absorption axis and is stretchable in the direction of the transmission axis. The upper limit of the strain amount can be, for example, 100%.

The shrinkage ratio of the polarizing film in the direction perpendicular to the absorption axis direction (transmission axis direction) when heated at 85° C. for 120 minutes is preferably 4.0% or less, more preferably 3.5% or less. If the shrinkage ratio is within such a range, it is possible to obtain a polarizing film that is prevented from breaking along the absorption axis direction and has stretchability in the transmission axis direction.

The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, still more preferably 5 μm or less, particularly preferably 3 μm or less, and particularly preferably 2 μm or less. The lower limit of the thickness of the polarizing film can be, for example, 1 μm. The thickness of the polarizing film may be 2 μm to 6 μm in one embodiment, 2 μm to 4 μm in another embodiment, and 2 μm to 3 μm in yet another embodiment. By making the thickness of the polarizing film very thin in this way, the amount of shrinkage due to heating can be made very small. It is speculated that such a configuration can also contribute to suppression of breakage in the direction of the absorption axis.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 40.0% or more, more preferably 41.0% or more. The upper limit of single transmittance can be, for example, 49.0%. The single transmittance of the polarizing film is 40.0% to 45.0% in one embodiment. The polarization degree of the polarizing film is preferably 99.0% or more, more preferably 99.4% or more. The upper limit of the degree of polarization can be, for example, 99.999%. The polarization degree of the polarizing film is 99.0% to 99.9% in one embodiment. According to the present invention, it is possible to realize a polarizing film having the amount of strain in the direction of the absorption axis and the heat shrinkage rate, and having practically acceptable single transmittance and degree of polarization. This is presumed to be due to the manufacturing method described later. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction. The single transmittance is a value when the refractive index of one surface of the polarizing plate is converted to 1.50 and the refractive index of the other surface is converted to 1.53. The degree of polarization is typically determined by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc, which are measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

The polarizing film is composed of a PVA-based resin film containing a dichroic substance, as described above. Preferably, the PVA-based resin constituting the PVA-based resin film (substantially, the polarizing film) contains an acetoacetyl-modified PVA-based resin. With such a configuration, a polarizing film having desired mechanical strength can be obtained. The amount of the acetoacetyl-modified PVA-based resin is preferably 5-20% by weight, more preferably 8-12% by weight, when the total PVA-based resin is 100% by weight. . If the blending amount is within such a range, a polarizing film having better mechanical strength can be obtained.

A polarizing film can typically be produced using a laminate of two or more layers. A specific example of a polarizing film obtained using a laminate is a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate. A polarizing film obtained by using a laminate of a resin base material and a PVA-based resin layer formed by coating on the resin base material can be obtained, for example, by applying a PVA-based resin solution to the resin base material and drying the resin base material. forming a PVA-based resin layer thereon to obtain a laminate of a resin substrate and a PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film; obtain. In this embodiment, preferably, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution for stretching. Furthermore, the stretching preferably further includes in-air stretching of the laminate at a high temperature (eg, 95° C. or higher) before stretching in an aqueous boric acid solution. In addition, the laminate is preferably subjected to drying shrinkage treatment to shrink the laminate by 2% or more in the width direction by heating while conveying in the longitudinal direction. The total draw ratio is preferably 2.5 times to 4.5 times. Even with such a total draw ratio, a polarizing film having acceptable optical properties can be obtained by combining the halide addition and drying shrinkage treatment. In one embodiment, a method for manufacturing a polarizing film includes subjecting a laminate to an auxiliary air-stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing auxiliary stretching, it is possible to improve the crystallinity of PVA and achieve high optical properties even when PVA is applied onto a thermoplastic resin substrate. At the same time, by increasing the orientation of PVA in advance, it is possible to prevent problems such as deterioration of orientation and dissolution of PVA when immersed in water in the subsequent dyeing process or stretching process, resulting in high optical properties. can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, disturbance of the orientation of the polyvinyl alcohol molecules and deterioration of the orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by drying shrinkage treatment. The obtained resin substrate/polarizing film laminate may be used as it is (that is, the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled off from the resin substrate/polarizing film laminate. Then, any appropriate protective layer may be laminated on the release surface according to the purpose. Details of the manufacturing method of the polarizing film will be described later in section C.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 has a polarizing film 10, a first protective layer 20 arranged on one side of the polarizing film 10, and a second protective layer 30 arranged on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in section A above. One of the first protective layer 20 and the second protective layer 30 may be omitted. As described above, one of the first protective layer and the second protective layer may be the resin base material used for manufacturing the polarizing film.

The first and second protective layers are formed of any suitable film that can be used as protective layers for polarizing films. Specific examples of the material that is the main component of the film include cellulose-based resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyimide-based, polyethersulfone-based, and polysulfone-based resins. , polystyrene-based, polynorbornene-based, polyolefin-based, (meth)acrylic-based, and acetate-based transparent resins. Thermosetting resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone, or ultraviolet curable resins may also be used. In addition, for example, a glassy polymer such as a siloxane-based polymer can also be used. Further, polymer films described in JP-A-2001-343529 (WO01/37007) can also be used. Materials for this film include, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and nitrile group in a side chain. can be used, for example, a resin composition comprising an alternating copolymer of isobutene and N-methylmaleimide and an acrylonitrile/styrene copolymer. The polymer film can be, for example, an extrudate of the resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 100 μm or less. 5 μm to 80 μm, more preferably 10 μm to 60 μm. In addition, when the surface treatment is performed, the thickness of the outer protective layer is the thickness including the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the protective layer (inner protective layer) disposed on the display panel side preferably has a thickness of 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. be. In one embodiment, the inner protective layer is a retardation layer with any suitable retardation value. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110 nm to 150 nm. “Re(550)” is an in-plane retardation measured with light having a wavelength of 550 nm at 23° C., and is obtained by the formula: Re=(nx−ny)×d. Here, “nx” is the refractive index in the direction in which the in-plane refractive index is maximized (that is, the slow axis direction), and “ny” is the in-plane direction orthogonal to the slow axis (that is, the fast is the refractive index in the axial direction), 'nz' is the refractive index in the thickness direction, and 'd' is the layer (film) thickness (nm).

C. Method for producing polarizing film A method for producing a polarizing film according to one embodiment of the present invention comprises polyvinyl alcohol containing a halide and a polyvinyl alcohol resin (PVA resin) on one side of a long thermoplastic resin substrate. By forming a system resin layer (PVA system resin layer) to form a laminate, and by subjecting the laminate to an auxiliary stretching process in the air, a dyeing process, an underwater stretching process, and heating while conveying in the longitudinal direction. and a drying shrinkage treatment that shrinks 2% or more in the width direction, and the total stretching ratio of the air auxiliary stretching treatment and the underwater stretching treatment is preferably 2 with respect to the original length of the laminate. 0.5 times to 4.5 times. The content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60°C to 120°C. The shrinkage ratio in the width direction of the laminate due to drying shrinkage treatment is preferably 2% or more. According to such a manufacturing method, the polarizing film described in section A above can be obtained. In particular, a laminate containing a PVA-based resin layer containing a halide is produced, the laminate is stretched in multiple stages including auxiliary stretching in the air and stretching in water, and the laminate after stretching is heated with a heating roll. , a polarizing film having excellent optical properties (typically, single transmittance and degree of polarization) can be obtained.

C-1. Production of Laminate Any appropriate method can be adopted as a method for producing a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin.

Any appropriate method can be adopted as a method for applying the coating liquid. Examples thereof include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The coating/drying temperature of the coating liquid is preferably 50° C. or higher.

The thickness of the PVA-based resin layer is preferably 2 μm to 30 μm, more preferably 2 μm to 20 μm. Reducing the thickness of the PVA-based resin layer before stretching to such a small thickness and making the total stretching ratio smaller than usual, as described later, will allow the PVA-based resin layer to exhibit its stretchability. It can contribute to the realization of a polarizing film having practically acceptable single transmittance and degree of polarization in spite of the small degree of orientation.

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be surface-treated (for example, corona treatment), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

C-1-1. Thermoplastic Resin Substrate Any appropriate thermoplastic resin film can be employed as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described, for example, in JP-A-2012-73580. The publication is incorporated herein by reference in its entirety.

C-1-2. Coating Liquid The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution in which the halide and the PVA-based resin are dissolved in a solvent. Examples of solvents include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Among these, water is preferred. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight with respect to 100 parts by weight of the solvent. With such a resin concentration, it is possible to form a uniform coating film in close contact with the thermoplastic resin substrate.

Additives may be added to the coating liquid. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyhydric alcohols such as ethylene glycol and glycerin. Surfactants include, for example, nonionic surfactants. These can be used for the purpose of further improving the uniformity, dyeability and stretchability of the obtained PVA-based resin layer.

Any appropriate resin can be adopted as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. An ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The degree of saponification of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, more preferably 99.0 mol% to 99.93 mol%. . The degree of saponification can be determined according to JIS K 6726-1994. By using a PVA-based resin having such a degree of saponification, a polarizing film with excellent durability can be obtained. If the degree of saponification is too high, gelation may occur. As described above, the PVA-based resin preferably contains an acetoacetyl-modified PVA-based resin.

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is usually 1,000 to 10,000, preferably 1,200 to 4,500, more preferably 1,500 to 4,300. The average degree of polymerization can be determined according to JIS K 6726-1994.

Any appropriate halide can be employed as the halide. Examples include iodide and sodium chloride. Iodides include, for example, potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred.

The amount of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the PVA resin, and more preferably 10 parts by weight to 15 parts by weight with respect to 100 parts by weight of the PVA resin. Department. If the amount of the halide exceeds 20 parts by weight with respect to 100 parts by weight of the PVA-based resin, the halide may bleed out and the finally obtained polarizing film may become cloudy.

In general, when the PVA-based resin layer is stretched, the orientation of the polyvinyl alcohol molecules in the PVA-based resin increases. Orientation may be disturbed and the orientation may be lowered. In particular, when a laminate of a thermoplastic resin substrate and a PVA-based resin layer is stretched in boric acid water, the laminate is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin substrate. When the film is stretched, the tendency of the degree of orientation to decrease is remarkable. For example, the stretching of a single PVA film in boric acid water is generally carried out at 60° C., whereas the stretching of a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is It is carried out at a high temperature of about 70° C., and in this case, the orientation of PVA at the initial stage of stretching may be lowered before it is increased by stretching in water. On the other hand, by preparing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate and stretching the laminate at a high temperature (auxiliary stretching) in air before stretching in boric acid water, , the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, the disturbance of the orientation of the polyvinyl alcohol molecules and the deterioration of the orientation can be suppressed compared to the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid.

C-2. Aerial Auxiliary Stretching In order to obtain particularly high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid solution is selected. By introducing auxiliary stretching, such as two-stage stretching, it is possible to stretch while suppressing crystallization of the thermoplastic resin base material. Furthermore, when applying the PVA-based resin on the thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, compared to the case of applying the PVA-based resin on a normal metal drum As a result, the crystallization of the PVA-based resin becomes relatively low, which may cause a problem that sufficient optical properties cannot be obtained. On the other hand, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when the PVA-based resin is applied onto a thermoplastic resin substrate, thereby achieving high optical properties. It becomes possible. At the same time, by increasing the orientation of the PVA-based resin in advance, it is possible to prevent problems such as deterioration of the orientation and dissolution of the PVA-based resin when it is immersed in water in the subsequent dyeing process or stretching process. , making it possible to achieve high optical properties.

The stretching method of the in-air auxiliary stretching may be fixed edge stretching (e.g., a method of stretching using a tenter stretching machine) or free edge stretching (e.g., a method of uniaxially stretching the laminate through rolls having different peripheral speeds). Although good, free-end drawing may be positively employed in order to obtain high optical properties. In one embodiment, the auxiliary in-air stretching process includes a heated roll stretching step in which the laminate is stretched by a peripheral speed difference between heated rolls while being conveyed in the longitudinal direction. The auxiliary aerial stretching process typically includes a zone stretching process and a hot roll stretching process. The order of the zone stretching process and the heating roll stretching process is not limited, and the zone stretching process may be carried out first, or the heating roll stretching process may be carried out first. The zone drawing step may be omitted. In one embodiment, the zone drawing step and the heated roll drawing step are performed in this order. In another embodiment, in a tenter stretching machine, the film is stretched by gripping the ends of the film and increasing the distance between the tenters in the machine direction (the extension of the distance between the tenters is the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the machine direction) is set to be arbitrarily close. Preferably, the draw ratio in the machine direction can be set to be closer to the free end draw. In the case of free end stretching, the shrinkage ratio in the width direction is calculated by (1/stretching ratio) ^1/2 .

The in-air auxiliary stretching may be performed in one step or in multiple steps. When it is carried out in multiple stages, the draw ratio is the product of the draw ratios in each step. The stretching direction in the in-air auxiliary stretching is preferably substantially the same as the stretching direction in the underwater stretching.

The draw ratio in the in-air auxiliary drawing is preferably 1.5 to 4.0 times, more preferably 1.7 to 3.5 times, still more preferably 2.0 to 3.0 times. be. If the stretch ratio of the in-air auxiliary stretching is within such a range, the total stretching ratio can be set within a desired range when combined with the underwater stretching. As a result, it is possible to obtain a stretchable polarizing film in which breakage along the direction of the absorption axis is suppressed.

The stretching temperature for the in-air auxiliary stretching can be set to any appropriate value depending on the material for forming the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably the glass transition temperature (Tg) of the thermoplastic resin substrate or higher, more preferably the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C or higher, and particularly preferably Tg + 15°C or higher. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, it is possible to suppress rapid crystallization of the PVA-based resin and suppress problems caused by the crystallization (for example, hindrance of orientation of the PVA-based resin layer due to stretching). can.

C-3. Insolubilization Treatment, Dyeing Treatment, and Crosslinking Treatment If necessary, an insolubilization treatment is performed after the auxiliary stretching treatment in the air and before the stretching treatment in water or the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The dyeing treatment is typically performed by dyeing the PVA-based resin layer with a dichroic substance (typically iodine). If necessary, a cross-linking treatment is performed after the dyeing treatment and before the underwater stretching treatment. The cross-linking treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. Details of the insolubilization treatment, dyeing treatment and cross-linking treatment are described, for example, in JP-A-2012-73580 (above).

C-4. Underwater Stretching Treatment Underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the underwater stretching treatment, the thermoplastic resin substrate and the PVA-based resin layer can be stretched at a temperature lower than the glass transition temperature (typically, about 80° C.), and the PVA-based resin layer undergoes its crystallization. can be stretched while suppressing As a result, a polarizing film having excellent optical properties can be produced.

Any appropriate method can be adopted as a method for stretching the laminate. Specifically, fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching a laminate by passing it between rolls having different peripheral speeds) may be used. Free-end drawing is preferably chosen. The laminate may be stretched in one step or in multiple steps. In multistage stretching, the total stretching ratio is the product of the stretching ratios in each stage.

The stretching in water is preferably carried out by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). By using an aqueous boric acid solution as the stretching bath, the PVA-based resin layer can be imparted with rigidity to withstand tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can form a tetrahydroxyborate anion in an aqueous solution and crosslink with a PVA-based resin through hydrogen bonding. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, which can be satisfactorily stretched, and a polarizing film having excellent optical properties can be produced.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate salt in water as a solvent. The boric acid concentration is preferably 1 part by weight to 10 parts by weight, more preferably 2.5 parts by weight to 6 parts by weight, and particularly preferably 3 parts by weight to 5 parts by weight with respect to 100 parts by weight of water. is. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher properties can be produced. In addition to boric acid or borate salts, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

Preferably, an iodide is added to the drawing bath (boric acid aqueous solution). By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. Specific examples of iodides are as described above. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, per 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such a temperature, the film can be stretched at a high magnification while suppressing dissolution of the PVA-based resin layer. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60° C. or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40° C., it may not be possible to stretch well even if the plasticization of the thermoplastic resin base material by water is considered. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, which may make it impossible to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The draw ratio in the underwater drawing is preferably 1.0 to 3.0 times, more preferably 1.0 to 2.0 times, still more preferably 1.0 to 1.5 times. . If the stretching ratio in the underwater stretching is in such a range, the total stretching ratio can be set within a desired range. As a result, it is possible to obtain a stretchable polarizing film in which breakage along the direction of the absorption axis is suppressed. The total stretching ratio (the product of each stretching ratio when the auxiliary stretching in the air and the stretching in water are combined) is, as described above, preferably 1.5 to 4.7 times the original length of the laminate. Yes, more preferably 2.5 to 4.5 times. In a general method for producing a polarizing film, the total draw ratio is 5.0 times or more, preferably 5.5 times or more, but in the embodiment of the present invention, the draw ratio is lower than this. This suppresses the high orientation of the PVA-based resin layer. As a result, breakage along the absorption axis direction of the obtained polarizing film can be suppressed, and stretchability (in particular, stretchability in the absorption axis direction) can be imparted. In addition, by appropriately combining the addition of a halide to the coating solution, the adjustment of the stretch ratios of the auxiliary stretching in the air and the stretching in water, and the drying shrinkage treatment, it is possible to obtain a polarizing film even at such a total stretching ratio. The optical properties can be made within a practically acceptable range. In one embodiment, the ratio of the draw ratio in the auxiliary drawing in air to the draw ratio in the drawing in water (the draw ratio in the auxiliary drawing in the air/the draw ratio in the water drawing) can be, for example, 1.14 to 3.5.

C-5. Drying Shrinkage Treatment The drying shrinkage treatment may be performed by zone heating performed by heating the entire zone, or by heating the transport roll (using a so-called heating roll) (heated roll drying method). Preferably both are used. By drying using a heating roll, it is possible to efficiently suppress heat curling of the laminate and produce a polarizing film having an excellent appearance. Specifically, by drying the laminated body along the heating roll, the crystallization of the thermoplastic resin substrate can be efficiently promoted to increase the degree of crystallinity, which is relatively low. Even at the drying temperature, the degree of crystallinity of the thermoplastic resin substrate can be favorably increased. As a result, the thermoplastic resin base material has increased rigidity and is in a state capable of withstanding shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Moreover, by using a heating roll, the layered product can be dried while being maintained in a flat state, so that not only curling but also wrinkling can be suppressed. At this time, the laminate can be shrunk in the width direction by drying shrinkage treatment, thereby improving the optical properties. This is because the orientation of PVA and PVA/iodine complex can be effectively enhanced. The shrinkage ratio of the laminate in the width direction due to drying shrinkage treatment is preferably 2% or more, more preferably 2% to 8%, and particularly preferably 4% to 6%.

FIG. 2 is a schematic diagram showing an example of drying shrinkage treatment. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls R1 to R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate. The transport rolls R1 to R6 may be arranged so as to continuously heat only the surface of the resin base material.

The drying conditions can be controlled by adjusting the heating temperature of the transport rolls (the temperature of the heating rolls), the number of heating rolls, the contact time with the heating rolls, and the like. The temperature of the heating roll is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. The degree of crystallinity of the thermoplastic resin can be favorably increased, curling can be favorably suppressed, and an optical laminate having extremely excellent durability can be produced. The temperature of the heating roll can be measured with a contact thermometer. In the illustrated example, six transport rolls are provided, but there is no particular limitation as long as the number of transport rolls is plural. Conveying rolls are usually 2 to 40, preferably 4 to 30 in number. The contact time (total contact time) between the laminate and the heating roll is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, still more preferably 1 to 10 seconds.

The heating roll may be provided in a heating furnace (for example, oven), or may be provided in a normal production line (under room temperature environment). Preferably, it is provided in a heating furnace equipped with air blowing means. By using both heating roll drying and hot air drying, abrupt temperature changes between the heating rolls can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature for hot air drying is preferably 30°C to 100°C. Moreover, the hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10m/s to 30m/s. The wind speed is the wind speed in the heating furnace and can be measured with a mini-vane digital anemometer.

C-6. Other Treatments Preferably, a washing treatment is performed after the underwater stretching treatment and before the drying shrinkage treatment. The cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of potassium iodide.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples. The measurement method of each characteristic is as follows. "Parts" and "%" in Examples and Comparative Examples are by weight unless otherwise specified.
(1) Thickness Measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name: "MCPD-3000").
(2) Single transmittance and degree of polarization The polarizing film/resin substrate laminates (polarizing plates) obtained in Examples and Comparative Examples were measured using an ultraviolet-visible spectrophotometer (LPF200 manufactured by Otsuka Electronics Co., Ltd.). Single transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc were defined as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp and Tc are Y values measured with a 2-degree field of view (C light source) according to JIS Z8701 and subjected to visibility correction. From the obtained Tp and Tc, the degree of polarization was determined using the following formula.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100
(3) Strain amount (elongation rate)
The amount of strain was measured using "TMA/SS 6100" (maximum load of about 5 N) manufactured by Hitachi High-Tech Science. Specifically, the polarizing films obtained in Examples and Comparative Examples were cut to 2 mm in the width direction and 25 mm in the length direction, and the strain amount (original The ratio of elongation to length) was measured.
(4) Shrinkage by heating The shrinkage was measured using “TMA Q-400” manufactured by TA Instruments. Specifically, the polarizing films obtained in Examples and Comparative Examples were cut to 4 mm in the width direction and 35 mm in the longitudinal direction. It was heated to ° C. and held at 85 ° C. for 2 hours, then lowered to 20 ° C. at a cooling rate of 10 ° C./min. / original length x 100).

[Example 1]
A long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used as the thermoplastic resin substrate, and one side of the resin substrate was subjected to corona treatment.
Polyvinyl alcohol (degree of polymerization: 4,200, degree of saponification: 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOSEFIMER") were mixed at a ratio of 9:1, and 100 parts by weight of PVA-based resin. was added with 13 parts by weight of potassium iodide and dissolved in water to prepare an aqueous PVA solution (coating solution).
The above PVA aqueous solution was applied to the corona-treated surface of the resin base material and dried at 60° C. to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.
The resulting laminate was uniaxially stretched 2.4 times in the machine direction (longitudinal direction) in an oven at 130° C. (in-air auxiliary stretching treatment).
Next, the laminate was immersed in an insolubilizing bath (an aqueous boric acid solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the finally obtained polarizing film is placed in a dyeing bath (iodine aqueous solution obtained by blending iodine and potassium iodide at a weight ratio of 1:7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ° C. It was immersed for 60 seconds while adjusting the concentration so that the single transmittance (Ts) was 42.3% (dyeing treatment).
Next, it was immersed for 30 seconds in a cross-linking bath at a liquid temperature of 40°C (an aqueous solution of boric acid obtained by blending 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water). (crosslinking treatment).
After that, the laminate is immersed in an aqueous boric acid solution (boric acid concentration: 4% by weight, potassium iodide concentration: 5% by weight) at a liquid temperature of 70° C., and stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds. The film was uniaxially stretched so that the total draw ratio was 3.0 times (underwater drawing treatment: the draw ratio in the water drawing treatment was 1.25 times).
After that, the laminate was immersed in a washing bath (aqueous solution obtained by blending 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (washing treatment).
After that, while drying in an oven kept at 90° C., it was brought into contact with a heating roll made of SUS whose surface temperature was kept at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%.
Thus, a polarizing film having a thickness of 7.2 μm (Ts: 42.3%, degree of polarization: 99.89%) was formed on the resin substrate.

[Example 2]
A polarizing film (thickness: 6.2 μm, Ts : 42.3%, degree of polarization: 99.98%).

[Example 3]
A polarizing film (thickness: 6.1 μm, Ts : 42.4%, degree of polarization: 99.99%).

[Example 4]
A polarizing film (thickness: 6.0 μm, Ts : 42.2%, degree of polarization: 99.99%).

[Comparative Example 1]
A polarizing film (thickness: 5.5 μm, Ts : 42.3%, degree of polarization: 99.99%).

For the polarizing films obtained in the above examples and comparative examples, the amount of strain when pulled in the direction of the absorption axis and the heat shrinkage rate in the direction of the absorption axis were measured. Results are shown in Table 1 and FIG.

As is clear from Table 1 and FIG. 3, the polarizing films of Examples are more resistant to heat shrinkage in the direction of the absorption axis than the polarizing films of Comparative Examples (as a result, breakage along the direction of the absorption axis is suppressed). ) and has stretchability in the absorption axis direction. In addition, as described above, the polarizing films of the examples have practically acceptable individual transmittance and degree of polarization.

Further, for the polarizing films obtained in the above Examples and Comparative Examples, the polarizing film was cut to 25 mm in the width direction and 2 mm in the length direction, and was stretched in the width direction (transmission axis direction). When the amount of strain (elongation rate) in the transmission axis direction was measured in the same manner as the measurement of the amount, the amounts of strain of the polarizing films of Examples 1 and 2 were 52% and 53%, respectively. On the other hand, the polarizing film of Comparative Example 1 was broken during the measurement and could not be measured.

The polarizing film and polarizing plate of the present invention are preferably used for, for example, a liquid crystal display (wearable device).

REFERENCE SIGNS LIST 10 polarizing film 20 first protective layer 30 second protective layer 100 polarizing plate

Claims (4)

Composed of a polyvinyl alcohol resin film containing a dichroic material,
A single transmittance of 40.0% or more and a degree of polarization of 99.0% or more,
It has an elongation of 10% or more when pulled in the direction of the absorption axis at a load change rate of 98.0 mN/min, and a shrinkage rate of 5% or less in the direction of the absorption axis when heated at 85°C for 120 minutes. , a method for manufacturing a polarizing film,
Forming a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to form a laminate, and
The laminate is subjected to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment for shrinking the laminate by 2% or more in the width direction by heating while being transported in the longitudinal direction, in this order. including
The total stretching ratio of the air auxiliary stretching treatment and the underwater stretching treatment is 3.0 to 4.5 times the original length of the laminate,
The production method, wherein the ratio of the draw ratio in the air auxiliary drawing treatment to the draw ratio in the underwater drawing treatment is 1.14 to 3.5. 2. The method for producing a polarizing film according to claim 1, wherein the polarizing film has a thickness of 8 [mu]m or less. 3. The method for producing a polarizing film according to claim 1, wherein the polarizing film has a single transmittance of 41.0% or more and a degree of polarization of 99.4% or more. obtaining a polarizing film by the method for producing a polarizing film according to any one of claims 1 to 3; and
A method of manufacturing a polarizing plate, comprising disposing a protective layer on at least one side of the polarizing film.

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